A conventional BWR comprises a reactor pressure vessel containing reactor coolant which removes heat from the nuclear core. BWRs use high-purity water as the neutron moderator and primary coolant in the production of steam. Respective piping circuits carry the heated water or steam to the steam generators or turbines and carry circulated water or feedwater back to the vessel.
Stress corrosion cracking (SCC) is a known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds, exposed to high-temperature (i.e., 150.degree. C. or higher) water. SCC refers to cracking propagated by static or dynamic tensile stressing in combination with corrosion at the crack tip. It is well known that SCC occurs at higher rates when oxygen is present in the reactor water in concentrations of about 5 ppb or greater. SCC is further increased in a high radiation flux where oxidizing species, such as oxygen, hydrogen peroxide, and shortlived radicals, are produced from radiolytic decomposition of the reactor water.
In a BWR, the radiolysis of the primary water coolant in the reactor core causes the net decomposition of a small fraction of the water to the chemical products H.sub.2, H.sub.2 O.sub.2, O.sub.2 and oxidizing and reducing radicals. For steady-state operating conditions, equilibrium concentrations of O.sub.2, H.sub.2 O.sub.2, and H.sub.2 are established in both the water which is recirculated and the steam going to the turbine. As a result of water radiolysis, the coolant in the BWR under normal water chemistry (NWC) operation conditions contains approximately 200 ppb of oxidants (O.sub.2 +H.sub.2 O.sub.2) in the recirculation line and several hundred ppb of oxidants in the core region. This range of oxidant concentration increases the potential susceptibility of austenitic stainless steel and certain nickelbase alloys to intergranular stress corrosion cracking (IGSCC) when other requisite factors such as stress and sensitization are present.
One method employed to mitigate IGSCC of susceptible material is the application of hydrogen water chemistry (HWC), whereby the oxidizing nature of the BWR environment is modified to a more reducing condition. This effect is achieved by adding hydrogen gas to the reactor feedwater. When the hydrogen reaches the reactor vessel, it reacts with the radiolytically formed oxidizing species to reform water, thereby lowering the concentration of dissolved oxidizing species in the water in the vicinity of metal surfaces. The rate of these recombination reactions is dependent on local radiation fields, water flow rates and other variables. Corrosion potentials of austenitic stainless steels and certain nickel-base alloys in contact with reactor water containing oxidizing species can be decreased below a critical potential by injection of hydrogen into the water so that the dissolved concentration is about 50 ppb or greater. For adequate feedwater hydrogen addition rates, conditions necessary to inhibit IGSCC can be established in certain locations of the reactor. Different locations in the reactor system require different levels of hydrogen addition.
In some reactors after switching from NWC to HWC, an increase in recirculation piping dose rates has been observed, but other plants have shown very minimal or no effect. For those reactors showing a significant increase in dose rates, the piping contamination also appears to be more resistant to the conventional chemical decontamination processes. Consequently, in some cases, reactor operators are forced to reschedule the maintenance tasks to avoid high radiation exposures to skilled workers. Although the exact reasons for poor decontamination are not known, it has been suggested that a Cr-enriched oxide film can be formed on stainless steel surfaces under HWC conditions. This Cr-enriched oxide film probably provides the necessary protection of base metal from corrosion, but it may also provide additional adsorption and/or reaction sites for Co-60 deposition on stainless steel surfaces.
In a paper entitled "Effects of HWC on Radiation Field Buildup in BWRs", Int'l. Conf. Chemistry in Reactors, Apr. 24-27, 1994, Nice, France, Lin et al. reported gamma scan results obtained in a number of reactors. Evaluation of these results revealed that the Cr-51 activity was measured at much higher levels under HWC conditions than under NWC conditions. Under NWC conditions, Cr-51 is present in reactor water in anionic forms, most likely HCrO.sub.4.sup.-, at approximately 1-5 .mu.Ci/kg. Even at this high concentration, very little Cr-51 activity has been measured on piping surfaces simply because the solubilities of Cr.sup.+6 compounds are just too high under NWC conditions. Under HWC conditions the Cr-51 concentration in the reactor water generally drops down to &lt;0.1 .mu.Ci/kg. Although the chemistry environment in the core region may still be oxidizing, the out-of-core regions, particularly the recirculation lines and sample lines, may be very reducing, and therefore the Cr.sup.+6 ions are reduced to Cr.sup.+3 in more insoluble forms Cr.sub.2 O.sub.3 or Cr(OH).sub.3 and disappear in water by depositing on out-of-core surfaces, or even fuel surfaces in more reducing areas. In a few plants the Cr-51 activity was measured at 5-25 .mu.Ci/cm.sup.2 on recirculation pipe surfaces. By a simple calculation assuming the specific activity of Cr-51 is about 5 .mu.Ci/gm, the pipe surface is very likely to be covered by an extra layer of about 0.01 mg Cr/cm.sup.2 in the form of Cr.sub.2 O.sub.3. Although the piping surface oxide film was not analyzed, several crud samples taken from jet pump and some steam/turbine system surfaces in a reactor were analyzed. Both Cr and Cr-51 were measured in significant proportions. In addition to the Cr-enriched oxide film already created under HWC conditions, an extra layer of Cr deposition may quickly become a favorable adsorption surface for soluble radioisotopes such as Zn-65 and Co-60. Consequently, the dose rates are significantly increased in some plants.
Thus, it is desirable to remove as much of the Cr-enriched oxide film as possible from the recirculation piping of a BWR. However, the stable Cr-enriched oxides, including mixed oxides and chromite, may be more difficult for decontamination solution to dissolve with conventional decontamination processes. Thus, a process is needed which overcomes the Cr-enriched oxide problem for decontamination processes.